Despite clinical advances, colorectal cancer (CRC) remains a leading cause of cancer-related death throughout the world. As one of the greatest barriers to CRC cure is the inability to achieve a durable response to adjuvant chemotherapy after surgery, it is both timely and clinically relevant to dissect the mechanisms underlying therapeutic resistance. Cancer initiating cells (CICs) contribute to tumor angiogenesis, invasion/metastasis, and therapeutic resistance. We identified a novel phenomenon that chemotherapy activates cancer associated fibroblasts (CAFs), with release of extrinsic signals, mainly interleukin 17A (IL17A), which remodels the tumor microenvironment by increasing CRC CIC proliferation, migration, tumor growth, and chemoresistance. These effects were mitigated by IL17A inhibition both in vitro and in vivo, suggesting a potential target for intervention. IL-1 stimulation leads to recruitment of Act1 to IL-17R and elicits multiple proliferative and cell survival pathways (including NF-kB, JNK, p38, ERK) in colon epithelial cells, but the role of IL17A in CICs is uncharacterized. When evaluating IL17A in humans, we found expression varies among CRC patients and increased expression is associated with worse outcomes. These observations suggest IL17A influences patient outcome, but details of this effect are not understood. Based on this background, I will now investigate the role of a critical cytokine (IL17A) in CIC maintenance and therapeutic resistance. I hypothesize that IL17A, derived from activated CAFs in the tumor microenvironment, remodels the cellular hierarchy and promotes CIC maintenance and therapy resistance. In the first aim, I will elucidate the molecular mechanism by which the IL17R-Act1 axis enhances human CIC tumorigenesis and chemoresistance. In the second aim, I will determine the therapeutic benefit of targeting IL17A-induced signaling in CRC using human CICs in culture and in vivo mouse CRC models, employing combination therapies including chemotherapy, IL17 inhibition, and MEK/ERK inhibition. Lastly, I will explore the effects of IL17 in ex vivo human CRC specimens with correlation to genetic and clinical characteristics to develop a biomarker profile. The proposed work will provide insight to molecular mechanisms driving CRC treatment resistance, test novel treatment combinations, and identify an appropriate patient population suited for these treatments, all of which will lead toward the long-term goal of improving survival for CRC patients.